Unit 7 Computer-Aided Quality Control INTRODUCTION The quality control (QC) function has traditionally been performed using manual inspection methods and statistical sampling procedures.
Manual inspection is generally a time-consuming procedure which involves precise, yet monotonous work. If often requires that parts be removed from the vicinity of the production machines to a separate inspection area. This causes delays and often constitutes a bottleneck in the manufacturing schedule. Inherent in the use of statistical sampling procedures is acknowledgment of the risk that some defective parts will slip through. Indeed, statistical quality control attempts to guarantee that a certain expected or average fraction defect rate will be generated during the production/inspection process. The nature of traditional statistical QC procedures is that something less than 100% good quality must be tolerated. There is another aspect of the traditional QC inspection process which detracts from its usefulness. It is often performed after the fact. The measurements are taken and the quality is determined after the parts are already made. If the parts are defective, they must be scrapped or reworked at a cost which is often greater than their original cost to manufacture.
All of these various factors are driving the quality control function toward what we are calling computer-aided quality control (CAQC). Other terms that have been applied to describe this movement are "computer-aided inspection" (CAI) and "computer-aided t esting" (CAT). The objectives of computer-aided quality c ontrol are ambitious, yet straight forward. They are: 1. To improve product quality 2. To increase productivity in the inspection process 3. To increase productivity and reduce lead times in manufacturing The strategy for achieving these objectives is basically to automate the inspection process through the application of computers combined with advanced sensor technology. Wherever technically possible and economically feasible, inspection will be done on a 100% basis rather than sampling. TERMINOLOGY IN QUALITY CONTROL
Quality in a manufacturing context can be defined as the degree to which a product or its components conform to certain standards that have been specified by the designer. The design standards generally relate to the materials, dimensions and tolerances, appearance, performance, reliability, and any other measurable characteristic of the product. Quality assurance (QA) is concerned with those activities which will maximize the probability that the product and its components will be manufactured within the design specifications. These activities should start in the product design area, where the designer can make decisions among alternatives that might have quality consequences. QA activities continue in manufacturing planning, where decisions relative to production equipment, tooling, methods, and motivation of employees will all have an influence on quality. Quality control is concerned with those activities related to inspection of product and component quality, detection of poor quality, and corrective action necessary to eliminate poor quality. These activities also involve the planning of inspection procedures and the specification of the gages and measuring instruments i nstruments needed to perform the inspections. Statistical QC is generally divided into two categories: acceptance sampling and control charts. charts. Acceptance sampling is a procedure in which a sample is drawn from a batch of parts in order to assess the quality level of the batch and to determine whether the batch should be accepted or rejected. Acceptance sampling is based on the statistical notion that the quality of a random sample drawn from a larger population will be representative of the quality of that population. Control charts are used to keep a record over time of certain measured data colected from a process. A company would use control charts to monitor its own production
processes. The central line indicates the expected quality level of the process. The upper and lower control limits (UCL and LCL) are statistical measures of the variation in the process which would be tolerated without concluding that the process has erred. when these limits are exceeded, it usually means that something has changed the process, and an investigation should be initiated to deteimine the cause c ause
Both acceptance sampling and control charts can be applied to two situations in quality control: fraction defects and measured variables. In the fraction-defect case, the objective is to determine what proportion of the sample (and the population from which it came) are defective. This is often accomplished by a go/no go gage, which can quickly determine whether a part is within specification or not. In the measured-variable case, the object is to determine the value of the quality characteristic of interest (e.g., dimension, resistance, hardness, etc.). This requires the use of a measuring instrument of some kind (e.g., micrometer, ohmeter, hardness tester, etc.) and is normally a more time-consuming manual process than the go/no go case. Inspection is normally used to examine a component of a product in relation to the design standards specified for it. For a mechanical component, this would probably be concerned with the dimensions of the part. These might be checked with several go/no go gages or they might be measured with a micro meter and other instruments. The corfion situations that warrant inspection are: Incoming raw materials At various stages during manufacturing (e.g., when the parts are moved from one production department to another) At the completion of processing on the parts Before shipping the final assembled product to the customer Testing, Testing, on the other hand, is normally associated with the functional aspects of the item, and it is often directed at the final product rather than its components. In this usage, testing consists of the observation of the final product during operation under actual or simulated conditions. If the product passes the test, it is deemed suitable for sale. Several categories of tests used for final product evaluation: Simple functional tests under normal or simulated normal operating conditions Functional tests in which the product is tested under extreme (usually adverse conditions) Fatigue or wear tests tests to determine how long long the product product will function function until until Failure. Overload tests to determine the level of safety factor built into the product Environmental testing to determine how well the product will perform under different environments (e.g., humidity, temperature)
THE COMPUTER IN QC
CAI and CAT are performed automatically using the latest computer and sensor technology. Computer-assisted inspection and testing methods form only part, certainly a major part, of computeraided quality control. In our treatment of the subject we shall include the integration of the quality control function with CAD/CAM as a critical ingredient in the success of CAQC. The implications of the use of computer-aided quality control are important. The automated methods of CAQC will result in significant changes from the traditional concepts and methods. The following list will summarize the important effects likely to result from CAQC. 1. With CAI and CAT, inspection and testing will typically be accom plished on a 100% basis rather than by the sampling procedures normally used in traditional QC. 2. Inspection during production will be integrated into the manufacturing process rather than requiring that the parts be taken to some inspection area. This will help to reduce the elapsed time to complete the parts. On line will have to be accomplished in much less time than with current manual techniques. 3. The use of noncontact sensors will become much more widely used with computer-aided inspection. With contact inspection devices, the part must usually be stopped and often repositioned to allow the inspection device to be applied properly. With noncontact sensor devices, the part can often be inspected "on the fly." These devices, driven by the high-speed data processing capability of the computer, can complete the inspection in a small fraction of a second. 4. The on-line noncontact sensors will be utilized as the measurement component of computerized feedback control systems. These systems will be capable of making adjustments to the process variables based on analysis of the data collected by the sensors. Data would be plotted. This would not only allow out of tolerance conditions to be identified, but gradual shifts in the process could also be uncovered and corrective action taken. By regulating the process in this manner, parts will be made much closer to the desired nominal dimension rather than merely within tolerance. Quality feedback control systems will help to reduce scrap losses and improve product quality. 5. With computer-aided inspection technology, it may no longer be necessary to settle for less than perfection. 6. Robots will be used increasingly in future inspection applications 7. There will also be applications for the computer in quality assurance as well as QC. The CAD/CAM data base will be used to derive these various quality applications, 8. There will be CAI and CAT take its place, manual inspection activity will be reduced. Quality control personnel will have to become more computer-wise and technologically sophisticated to operate the more complex inspection and testing equipment and to manage the information that will result from these more automated methods. CONTACT INSPECTION METHODS The contact methods usually involve the use of coordinate measuring machines (CMM). Most of these machines today are either controlled by NC or computers. The coordinate measuring machine (CMM) is the most prominent example of the equipment used for contact inspection of workparts. It consists of a table which holds the part in a fixed, registered position and a movable head which holds a sensing probe. The probe can be moved in three directions, corresponding to the x, y, and z coordinates. During operation, the probe is brought into contact with the part surface to be measured and the three coordinate positions are indicated to a high level of accuracy. Today's coordinate measuring machines are computer controlled. The operation of the machine is similar to an NC machine tool in which the movement of the measuring probe is either tape controlled or computer controlled. Programs and coordinate data can be downloaded from a central computer, much in the manner of direct numerical control. Also similar to DNC is the capability to transmit data from the CMM back up to the host computer.
Savings in inspection time by using coordinate measuring machines are significant. Typically, between 5 and 10% of the time is required on a CMM com pared to traditional manual inspection methods. Other advantages include consistency in the inspection process from one part to the next which cannot be matched by manual inspection, and reductions in production delays to get approval of the first workpiece in a batch. The coordinate measuring machine is physically located away from the production machine, usually in a separate area of the shop. Accordingly, the parts must be transported from the pro duction area to the CMM. In fact, if inspection is required at several different stages of production, several moves witl be involved. One possible approach to overcome this problem is to use inspection probes mounted in the spindle of the machine tool. These inspection probes are contact sensing devices that operate with the machine tool much like the coordinate measuring machine. NONCONTACT INSPECTION METHODS The noncontact methods are divided into two categories for our purposes: 1. Optical a. Machine vision b. Scanning laser beam devices c. Photogrammetry 2. Non Optical a. Electric Fiel techniques b. Radiation techniques c. Ultrasonics NONCONTACT INSPECTION METHODS- OPTICALT The advantages of noncontact inspection are: 1. It usually eliminates the need to reposition the workpart. 2. Noncontact inspection is usually much faster t han contact inspection. 3. It eliminates mechanical wear encountered with the contacting inspection probe because it eliminates the probe. 4. It reduces potential danger to people, who must touch a hazardous material if contact inspection is used. 5. It removes the possibility of damage to the surface of a part which migh result during contact inspection. Machine vision
Other names given to these systems include microprocessor-based television and computer vision. The typical machine vision system consists of a TV camera, a digital computer, and an interface between them that functions as a preprocessor. The combination of system system hardware and software digitizes the picture and analyzes the image by comparing it with data stored in memory. The data are often in the form of a limited number of models of the objects which are to be inspected. There are several limitations of machine vision;
The first limitation is concerned with the problem of dividing the picture into picture elements. This is very similar to the problem encountered in the development of graphics terminals for computer-aided design. A second limitation is that the object in front of the camera must be capable of being divided into areas of contrasting lightness and darkness.
Third limitations is on the capability of machine vision systems recognize the object in the viewing area. Machine vision inspection problems can be divided into two categories: 1. Noncontact gaging of dimensions - Noncontact gaging in machine vision involves the inspection of part size and other features where it is not necessary to process the image of the entire part out line, only those portions that must be examined for dimensional accuracy. During setup for an inspection, a parts-training program is used to view the workpart of interest on a TV monitor. With the image in fixed position on the screen, the operator manipulates a cursor to define the edges of interest and to apply an appropriate scale factor to establish the correct units of measure. 2. Inspection based on pattern pattern recognition of object features - It is based on pattern recogition recogition techniques. In this category, the attributes of the object to be inspected are typically more subjective and in some respects more complicated than part dimensions. The machine vision pattern recognition process can be conceptualized as involving a comparison of features (for example, area, perimeter, and so on) between the object being inspected and the model of the object stored in computer memory.
Scanning laser beam devices The scanning laser beam device relies on the measurement of time rather than light, although a light sensor is required in its operation. The schematic diagram of its operation is pictured in Figure. A laser is used to project a continuous thin beam of light. A rotating mirror deflects the beam so that it sweeps across the object to be measured. The light sensor is located at the focal point of the lens system to detect the interruption of the light beam as it is blocked by the object. The time lapse corresponding to the interruption of the light beam is meas ured to determine the desired dimension of the part. Typically, a microprocessor is programmed to make the conversion of the time lapse into a dimensional value and to perform other functions, such as signaling an automatic parts-rejection mechanism to eject a defective part from the line.
Photogrammetry Photogrammetry involves the extraction of three-dimensional data from a pair of photographs taken at different angles. The two photographs can be combined much in the way that a stereoscope uses a pair of photographs to form a three-dimensional image for the viewer. In the measurement process used for inspection, the two photographs are read by a device called a monocomparator to establish coordinates and positions of objects. These data are then computeranalyzed to extricate the desired information. The drawback of the conventional photogrammetry technique is the need for photographs, an inconvenient and time-consuming step in the procedure. An improvement in the technique which is being developed will delete the photo graphic step. Instead, the images froiri two cameras set up in a stereoscopic configuration will send visual data directly to a computer for mathematical analysis and real-time extraction of dimensional data. T his arrangement is illustrated in Figure.
NONCONTACT INSPECTION METHODS@ NONOPTICAL In addition to noncontact inspection methods based on optical systems, nonoptical approaches can also be used. We will describe three general types which are quite representative of the current technology in this area. The three general types are: 1. Electrical field techniques 2. Radiation techniques 3. Ultrasonics Electrical field techniques Various types of electrical field techniques can be applied to noncontact inspection. Three types of electrical fields are employed: 1. Reluctance - The reluctance transducers are proximity devices that indicate the presence and distance from the probe of a ferromagnetic substance. The obvious limitation of the device is that the object being inspected must be electromagnetic. 2. Capacitance - A capacitance-based transducer can also be used to measure the distance of an object from the face of a probe. The measurement is based on the variable capacitance from part/probe coupling. This capacitance is inversely proportional to the distance between the probe face and the part, and thus the distance can be calculated. The capacitance transducer can be used to detect a variety of materials. The material must be an electrical conductor 3. Inductance - Inductance systems operate by subjecting the object to an alternating magnetic field by means of an electromagnetic coil. The result is that small circulating currents (eddy currents) are generated in the object. These eddy currents, in turn, create their own magnetic field, which interacts with the primary field. This interaction affects
the impedance of the coil, which an be measured and analyzed to determine certain characteristics about the object Radiation Techniques X-ray radiation techniques are employed for purposes of noncontact inspection in the metals and metalworking industry. The amount of radiation absorbed by a material can be used to measure its thickness and other quality characteristics. In a typical application in a rolling mill, an X-ray scanning unit measure the thickness of the plates or strips going through the rolls so that the proper adjustments can be made in the rollers. X-ray techniques are also used to inspect weld quality in fabricated steel and aluminum pressure vessels and pipes. In this case the radiation can be used to detect flaws and voids in the weld. Ultrasonics Ultrasonics in inspection work involves the use of very high frequency (above 20,000 Hz) sound waves to indicate quality. A principal application is in nondes tructive testing of materials. Ultrasonic techniques can also be applied to the problem of determining dimensional features of workparts. One approach, called acoustical phase monitoring, involves the analysis of sound waves reflected from the surface of an object. The sound waves are produced by an emitter and directed against the object. Assuming that all else remains constant, the reflected sound pattern from the object should always be the same. During inspection, the sound pattern from the part is analyzed by a computer program and compared to the pattern of a standard part, one that is known to be of acceptable quality. If the pattern of the test part differs significantly from that of the standard, it is rejected. COMPUTER-AIDED TESTING Testing is generally applied to assess the functional performance of a final product. It may also be applied for major subassemblies of the final product, such as the engines and transmissions of automobiles. Testing may also be performed on individual components in which some functional aspect of the component must be examined and cannot be implicitly determined by means of a mechanical inspection. An example of this might be the case of a brake lining in which the dimensions are correct, but the functional performance must be determined through a testing procedure. Computer-aided testing is simply the application of the computer in the testing procedure. There are different levels of automation which can be found found CAT. At the lowest level, the computer would be used simply to monitor the test and analyze the results, but the testing procedure itself is manually set up, initiated, and controlled by a human operator. In this case the computer receives the data from a data logger or a data acquisition system and prepares a report of the test results Computer-aided test cells are applied in situations where the product is complicated and produced in significant quantities, Examples include automobile engines, aircraft engines, and electronic integrated circuits. Advantages of these cells include higher throughput rates, greater consistency in the test procedure, and less floor space occupied by the automated cell
As compared to a manual facility of similar capacity.
INTEGRATION OF CAQC WITH CAD/CAM Although many important benefits result from the use of computer-aided quality control, additional benefits can be obtained by integrating CAQC with CAD/CAM.
The design department creates the product definition and the manufacturing department makes use of and supplements this definition to develop the manufacturing plan. It is important to add the QC connection to the CAD/CAM framework. The quality control department must use the same CAD/CAM data base to perform its function. These quality standards are all contained in the CAD/CAM data base, available for QC to use. One way in which the data base can be used is to develop the NC programs to operate the tape-controlled or computer-controlled coordinate measuring machines. These programs can be generated automatically. These programs would then be down loaded to the CMM through a DNC link from the central computer to the controller unit for the CMM. The same sort of downloading process is possible for some of the noncontact inspection methods. Another way in which a common data base is helpful to QC is when engineering changes are made to the product. It is helpful for any changes to be recorded in a common data file for all departments, including QC, to use. Another area where CAD/CAM benefits the QC function is in computer production monitoring. The types of production records that are generated during computer monitoring are sometimes useful to the quality control department in tracing the cause of poor quality in a particular production lot.
